Make it possible to call BIDS validator outside of full xnat2bids

[mirestrepo]

I think we should document how to run the validator independently of the whole conversion pipeline. https://docs.ccv.brown.edu/bnc-user-manual/xnat-to-bids-intro/bids-validation

Can we re-use the functions here even if we don't need to export anything?

and document in the user manual

Originally posted by @mirestrepo in #66 (review)

[elizabethlorenc]

copying my reply here: #66 (comment)